Introduction of alkali metals into evacuated containers



Patented Dec. 8, 1,931

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JOHN WESLEY MARDEN, OF EAST ORANGE, NEW JERSEY, assrerNoia .v T .wEs'rINe- HOUSE LAMP COMPANY, A CORPORATION. or PENNSYLVANIA INTRODUCTION OF ALKALI METALS INTO EVACUATEI) CONTAINERS No Drawing.

This invention relates to the introduction of chemically active metals into evacuated or gas-filled containers, such as electron discharge devices, and more particularly to the introduction of alkali metals into such devices.

In the manufacture of electric devices it is desirable to introduce therein, a small quantity of an active substance in a pure state, such as an alkali metal for the purpose of cleaning up residual gases in evacuated containers, purifying rare gases in gas-filled containers, activating the filament by increasing the electron emissivity thereof, lowering the starting and operating potential of discharge lamps, rectifiers, etc., creating a residual metallic vapor in the envelope and for various other purposes.

Extreme'difiiculty has hitherto been experienced in attempting to introduce these highly volatile materials, particularly the alkali metals, such as caesium, potassium, sodium, etc., into the envelopes of various gas tight containers, since in their free state, these metals combine very readily with the elements of the atmosphere and therefore cannot be handled to any extent.

The alkali metals caesium, potassium, sodium, lithium and rubidium, vaporize at relatively low temperatures and if they areintroduced in such devices priorto evacuation thereof, it is difficult to maintain them in a non-volatile state while baking out the envelope during the exhaust operation. Heretofore, the alkali metals have usually been introduced into the envelope after exhausting by vaporizing the same from a side tube or container.

Hitherto, it has been proposed to introduce volatile metals into evacuated containers in the form of a reducible compound which is subsequently reduced by means of a suitable reducing agent in order to liberate the alkali metal in the container. Such a method of introducing alkali metals is described in copending application Serial No. 8,865 of Duncan MacRae, filed February 13, 1925, and entitled Introduction of alkali metals.

According to the above application, a compound, such as a silicate, hydroxide or car- Application filed March '20, 1926. i Serial No. 96,335.

bonate of the metal, is mixed with a reducing agent and applied with. a suitable binder to a metallic part of the device which is adapted to be heated, as by high frequency induction, to effect the reduction of the compound.

.While this process produces good results there is danger of these substances absorbing water vapor prior to their introduction into the device which is liberated at the timethe decomposition takes place and deleteriously' effects the operation of the device in which the alkali metal is introduced. For instance, in electron discharge devices employing thoria'ted tungsten cathodes, the water vapor causes oxidation of the'thorium content of thecathode, destroying or seriously. impairing its emission. It has not been possible to heat these compounds, in baking the device sufficiently to eliminate allthe water therefrom. v In the manufacture of high vacuum devices it is the usualpractice tobake the electrodes in hydrogen prior'to-mounting toremove all traces of oxygen therefrom and during evacuation, and to bake out the envelope at relatively high temperatures to drive out all water vapor and occluded gaseslfrom the glass. It will be readily appreciated that it is useless to take these extreme precautions to eliminate all moisture from the device and toagain liberate water vaporafter the tube has been sealed off by the decomposition of the alkali metal compounds.

It is one of the objects of the present inventionto provide a practical method of introducing highly volatile metals, such as the alkali metal, into gas tight containers in 'a 'purestate, Without the simultaneous introduction of water vapor.

Another object is to provide a" method of introducing chemically active metals into evacuated containers in a non-hygroscopic condition and .free from deleterious substances. H

Another object is to provide an improved method of introducing alkali metal'compounds into gas tight containers and liberating the alkali1netal by reduction of the compound... 1

Other objects and advantages will hereinafter appear.

In accordance with this invention the chemically active metal, which it is desired to employ in the evacuated device, is introduced into the device in the form of a stable moisture free or anhydrous compound which is subsequently reduced to liberate the metal in pure form and free from water vapor or other substance which might deleteriously effeet the operation or life of the device. The stable compoundmay be mixed in powdered form with a fin'elydivided reducing agent, such as magnesium or aluminum and applied with a suitable binder to an interior portion of the device, capable of being heated to a sufficient temperature to effect the reduction of the compound.

If desired, the compound and reducing agent may be enclosed in a capsule such as that described in my copending application Serial No. 745,071, filed October 22, 192 entitled Introduction of vaporizable material into exhausted containers, and assigned to the l/Vesti nghouse Lamp Company. Briefly, such method consists inenclosing the compound in a metal tube of low melting point which is drawn to small size and cut into short lengths to form capsules containing a definite quantity of the material. The capsule may be welded or otherwise secured to a metal part of the device where it can be heated by high frequency induction currents to melt the capsule and liberate the contents.

The compounds which I prefer to employ are the double halide salts such as potassiumzirconium fluoride or caesium-uranium or zirconium fluoride, although the invention is not limited to any particular stable non-hygroscopic compounds since I have found that very satisfactory results can be obtained employing the permanganate, chromate or bichromate of the alkali metal. These latter compounds can be dried completely free of water and are easily prepared. -The double halide salts in powdered form may be mixed with a reducing agent such as magnesium or aluminum in molecular proportions or with an excess of reducing material and applied to the interior part of the device in any desired manner, where it may be subsequently heated to efiect a reaction. The magnesium or aluminum combines with the halide forming magnesium halide and liberating the alkali metal. When employing an alkali metal permanganate, such as caesium permanganate, I prefer to use a gentle reducing agent, such'aspowdered iron or nickel instead of aluminum and magnesium, since the permanganate contains a large quantity of oxygen and with powerful reducing agents, gives a vigorous reaction.

In the production of alkali metal radio tubes, such as caesium amplifier tubes which employ a tungsten filament, the combination ofcaesium permanganate or caesium bichromate and a metallic reducing agent, such as lron or aluminum, may render it possible to supply the oxygen for oxidizing a tungsten filament as well as supplying the caesium by one single heating process. In making caesium amplifier tubes at the present time, it is customary to thoroughly exhaust and bake out the tube and then to admit a small amount of oxygen therein from an outside container. The filament is flashed in the oxygen to oxidize the surface thereof to render it capable of retaining a film of caesium metal thereon. The excess oxygen is then pumped out and the caesium introduced into the device. 'By employing caesium permanganate or bichromate or other oxygen-eontaining compound of the alkali metal, the step of introducing oxygen from a separate source may be eliminated and the oxygen supplied by the decomposition of the compound. I

While a number of specific stable non-hygroscopic and moisture-free compounds have been specified hereimit is to be understood that. the invention is not limited to the compounds or reducing agents mentioned, but that the invention contemplates the use of all equivalent compounds which may be reducible either by heat alone or by other reducing agentsf In place of aluminu H1 and magnesium, nisch metal .may be employed for reducing the compound. Calcium metal may also be used, although the other reducing agents mentioned are preferred since calcium forms hyuroxides and carbonates in the 'air and therefore, admits of the possibility ofintroducing some water vapor or other deleterious material into the envelope.

It is understood that many modifications may be made in the invention described herein without departing therefrom and I do not desireto be limited to the exact embodiments described.

hat is claimed is The method of introducing caesium into an evacuated container which comprises introducing caesium-uranium fluoride mixed with magnesiuininto the container and heating the mixture to liberate the caesium.

2. A getter for an electric discharge device comprising a non-hygroscopic compound of caesium mixed with magnesium.

In testimony whereof, "I have hereunto subscribed my naine this 18th day of March,

onNWEs n MARDEN. 

